In a clad-pumping type fiber laser using a double-clad fiber, pump light that has not been absorbed in a Core (residual pump light) propagates through a cladding even in an output portion of a cavity. Particularly, because a high-power fiber laser is supplied with pump light having some power, residual pump light tends to be generated at an output portion of a cavity. Such residual pump light propagates in a multi-mode and thus has low beam quality. Accordingly, if such residual pump light is emitted from a fiber laser along with signal light, the quality of the output beam from the fiber laser is impaired, so that fine processing cannot be achieved with use of the output beam.
Furthermore, emission optics such as isolators or collimators are designed only for transmitting signal light in view of the controllability of the quality and the cost. Therefore, when residual pump light is introduced into those emission optics, unintentional optical absorption may occur to cause generation of heat or a serious accident such as fire.
Accordingly, residual pump light needs to be emitted to an exterior of an optical fiber before it reaches a laser output end. The following mechanism has been known as a mechanism for emitting such residual pump light to an exterior of an optical fiber. A covering material of a double-clad fiber is removed over its whole circumference so as to expose a cladding, and an exposed cladding is sandwiched between two optical substrates having a refractive index higher than that of the covering material (see, e.g., Patent Literature 1). When a cladding is thus sandwiched between optical substrates having a high refractive index, residual pump light can be emitted into the optical substrates at a contact area between the exposed portion of the cladding and the optical substrates.
With this structure, however, the cladding has line contact with the optical substrates. Therefore, this structure requires that the length of the line contact considerably be increased in order to sufficiently remove the residual pump light, and is thus inefficient. Additionally, since the covering material needs to be removed over a long distance, the cladding is likely to be damaged during the removal process of the covering material. Therefore, this structure lacks the reliability.
From this point of view, there has been proposed a structure as shown in FIG. 1. With the structure shown in FIG. 1, two double-clad fibers 510 and 610 are fused within a space surrounded by a reinforcement member 500. A downstream end of a covering material 520 of the double-clad fiber 510 is removed over its whole circumference. Thus, a cladding 530 is exposed from the covering material 520. Similarly, an upstream end of a covering material 620 of the double-clad fiber 610 is removed over its whole circumference, and a cladding 630 is thus exposed from the covering material 620. An exposed end of the cladding 530 of the double-clad fiber 510 and an exposed end of the cladding 630 of the double-clad fiber 610 are fused at a fusion splicing point 700.
A space around those exposed claddings 530 and 630 (and around the covering materials 520 and 630) is filled with a resin 540 having a refractive index that is equal to or higher than those of the claddings 530 and 630. With this structure, the exposed cladding 530 is held in contact with the resin 540 over its whole circumference. Thus, a large contact area between the cladding 530 and the resin 540 improves the efficiency of emitting residual pump light from the cladding 530 into the resin 540.
With the structure shown in FIG. 1, however, a portion of the covering material 520 is removed over its whole circumference. Therefore, residual pump light that has propagated through the cladding 530 and the core is locally emitted into the resin 540 at the most upstream part 542 of the fiber portion from which the covering material 520 has been removed. Accordingly, the residual pump light is locally absorbed in the resin 540 at the most upstream part 542, where the amount of generated heat is increased Such local heat generation may increase the temperature of a local area of the double-clad fiber 510, resulting in greatly lowered reliability.